The level around the plant's administration building rose to 4,000 microsievert per hour from 3,700 after the trucks joined an unprecedented attempt to cool down the reactor's apparently overheating fuel pool, after SDF helicopters dropped tons of water earlier in the day.

It was unchanged after the choppers dumped seawater onto the reactor shortly before 10 a.m., the utility said earlier in the day.

The level around the plant's quake-proof building at which workers are standing by had risen to about 3,000 microsievert per hour, it said in the morning. The level compares to 1,000 microsievert, or 1 millisievert, to which people can be safely exposed in one year.

Defense Minister Toshimi Kitazawa said he had given the go-ahead for the helicopters to drop water as the radiation level was 4.13 millisievert per hour at an altitude of 1,000 feet and 87.7 millisievert at 300 feet.

The choppers actually did so at a height of less than 300 feet, but their 10 crew members suffered no health problems with less than 60 millisievert of radiation measured from them after decontamination, against 100 millisievert to which they can be exposed in an emergency mission, SDF generals said."